Not even sure if you can even buy a non hybrid these days whether petrol or diesel they all have some sort of small battery and ISG before you even get to a plugin.
Just technology progression really, cars get better over time, obviously when I compare my PHEV to my diesel, my diesel was from 2010s so hardly the latest and greatest, the petrol now has direct injection, plus anything that used to hang off the engine parasitically is now detached/electric, more effort has also been spent on aero and rolling resistance etc to meet new targets, just how it is. Obviously having a charged battery for high consumption regions makes things a lot better.
Driving around the city in my 2 tonne SUV with a depleted battery was barely any worse than my old 1.1 tonne city car as it would do enough on the stop start in B mode to pretty much act like a hybrid, seemingly always enough charge to take up the start up /pull away load even if not to use for full drive.
My new city car is better on fuel now but its sub a tonne and 1.0, so quite a bit different, it can almost go in the boot of my SUV (joke, it’s an Up! No really that small ) but it’s not that much better really than the SUV and of course in the city the SUV when charged is just on electric, it’s very rare we forget to plug it in, more often than not it’s the case that you say to yourself that charge will be fine for tomorrow, don’t put it on charge and then something comes up and you have to do 200 miles or something, no biggy, big tank of fuel.
Of course it’s a compromise but if you know your average mileage you can calculate if it will work for you once you take out the fringe cases.
A diesel still has its place but regulations and poor rep mean that not many are pushing these.
Yup my manufacturer mpg is 283, nope, never gonna happen